1. Field of the Invention
This invention relates to amorphous silica product and process and is more particularly concerned with a process of using and products from fume amorphous silica particulates usually recovered from an electric furnace which produces metallic silicon.
2. Description of the Prior Art
In the past, amorphous silica slips have been produced by grinding in a ball mill, conventional amorphous (fused) silica and water. Such prior art amorphous fused silica was produced by melting sand or other quartz crystals. Such silica, when ground with water formed slips which were then cast into prescribed shapes which were then dried and fired. The grinding operation was necessary in order to activate the surface of amorphous fused silica sufficiently to capture and retain the water and cause bonding at the millions of contact points of the extremely fine particles when the slip was dried. When amorphous fused silica is ground dry, and then water added, the mixture does not bond to any appreciable extent and will not produce a useful product.
The U.S. Patent to Lindenthal U.S. Pat. No. 3,163,688 is typical of this prior art and shows preparing slip by grinding fused silica with water. As a practical matter, the major fabricators use about 17% water rather than Lindenthal's 28.57%, and grind perhaps 1/3 as long. This uses less power, with less capital tied up in equipment, and results in faster casting of the silica when poured into plaster of paris molds to cast into shapes. The slip castings are made for various end uses, usually with the addition in the slip of silica glass grains of sand like size and shape, up to 4 mesh size. This makes possible thick and complex castings.
Foam insulating blocks are being made from this slip by a very careful mixing, with a foam, such as firefighting foam, of water and proteins or special alcohols then cast in plaster molds. This takes many molds and much space and time. I tried to make these blocks by adding sawdust, wood flour, nut shell or peanut shell particles to the above slip, to be burned out in firing. Extrusions using the slip were not successful, and slip cast and pressed blocks cracked and are weak after firing.
The patent to Berner U.S. Pat. No. 2,047,457 teaches a procedure which is typical of the prior art efforts to impart plasticity to fused silica slip.
The slip produced from the prior art process is "dilatent" and does not lend itself to conventional clay shaping. Instead, it is usually cast.
It has, heretofore, been asserted in literature from the manufacturer that the fume silica stack particulates are "pozzolanic" and it has been suggested by such literature that such particulates be incorporated into concrete along with Portland cement to improve the properties of the concrete. I have found, however, that a block of fume silica, without the Portland cement is not pozzolanic, in that dried and unfired shapes of fume silica alone have low strength and will soften when wet.
The fume silica, i.e., the spherical amorphous silica stack particulates, and water mix of the present invention produce a plastic, rather than dilatent, product which is quite versatile and may be formed by dry pressing, ramming, extrusion, injection molding, potters wheel shaping or jiggering and may be machined either when partially dry or after firing.
The process of the present invention eliminates the necessity of (1) fusing quartz crystals and (2) grinding the resulting glass in water.
In the prior art a so-called "cement" is used as mortar and sometimes as a plaster facing for fused silica products, especially the foam block or insulating brick. This cement is made by mixing the fused silica slip of the prior art with silica glass grains and colloidal silica in water suspension. The cement is almost universally mixed at the silica fusion or fabrication plant and shipped to the point of use. With the vibrations of shipping, most of the solids in the cement settle out and form a hard cake which must then be remixed by prolonged rolling or tumbling or other tedious procedure.
This has not been avoided by delaying the mixing until after shipping, as the slip alone has the same settling problem.